Field studies of installed RCDs (also known as GFCIs) carried out in numerous countries such as the USA, Germany, Italy and Denmark, revealed that the level of defective RCDs found in installations in those countries varied from about 3% to 8%.
Electrical installation rules in most countries require RCDs to be tested at regular intervals, e.g. every 3 or 6 months. However, some of the above studies also revealed that in most cases, RCDs are rarely tested by users.
Undetected defective RCDs deprive users of shock and fire protection, but also create a potential risk and liability for responsible bodies or persons, e.g. business owners, managers of schools, hospitals, airports, etc. in the event of a defective RCD failing to operate when required. For those organizations that do undertake regular manual testing of RCDs, there is a growing economic cost of carrying out such tests.
The most obvious reason for failure of an RCD to operate is due to component failure. However, there are other reasons that an RCD could fail to operate, e.g. desensitization due to an installation fault such as a double grounding of the neutral (DGN) condition on an AC installation, or desensitization or even blinding of an AC type RCD due to the presence of DC currents flowing in an AC “system”, e.g. during electric vehicle charging.
For clarification, desensitization occurs when the trip level of an RCD increases above its rated operating level, e.g. when an RCD rated to trip at a residual current of 30 mA requires more than 30 mA to trip.
Many of the above problems are well known to those versed in the art of RCDs, which have been in use worldwide for about fifty years. However, for more detailed information on RCD technology, see the article “Demystifying RCDs” at www.westernautomation.com)
The use of RCDs with Self Test and End of Life indication has grown over recent years, and this has undoubtedly helped to improve electrical safety by exposing faulty RCDs. For convenience, the self test and end of life indication are collectively referred to as STEOL. Currently, the testing carried out under STEOL is limited to checking that the RCD can detect a residual current equivalent to a residual fault current. Generally, such ST testing does not require and even precludes the opening of the contacts. Failure of the contacts to open could be due to a variety of reasons such as welded contacts or mechanical or electrical factors associated with the opening means. However, it would be advantageous to have means to verify whether or not the contacts open under a fault condition, or whether they open under a Self Test condition where opening of the contacts is intended.